In recent years, in the automobile industry, car bodies have been reduced in weight in order to help meet CO2 emission limits. In view of this trend, securing strength and rigidity while lightening parts is being sought. To satisfy such demands, parts of members which have been made of metal in the past are being replaced with plastic to obtain parts made of metal and plastic members joined together. Employment of these is gradually increasing. In particular, progress is being made in development of carbon fiber reinforced plastics made of composites of plastic and carbon fiber (below, sometimes referred to as “CFRP”). Much is expected from them in the point of their securing strength and rigidity while lightening the weight of parts. For this reason, a joined part of a plastic member and CFRP member etc. and further a part joining a metal and CFRP or another plastic is also desired. Further, these joined parts may be utilized in the automotive field etc., so a high production efficiency, for example, a high joining speed and yield, is also being demanded.
In general, a metal is harder to join with not only CFRP, but also general plastics. Among these, as the method of joining a metal and a plastic member, there are the method of joining members using an adhesive etc., a method of providing a fastening member at least at one of the metal and the plastic member to fasten the two, a method of mechanically joining members using screws etc., a method of forming fine relief shapes at the joining surface on the metal side and joining the members by an anchor effect, a method of joining a metal and a plastic member by firing a laser beam at least at one of these, etc.
The methods of joining members using an adhesive, fastening members, screws, etc. require materials for joining a metal and a plastic member in addition to these members. However, from the viewpoint of improvement of economy and recyclability etc., the method of directly joining a metal and a plastic member is preferable. Further, the method of joining members by forming fine relief shapes at the joining surface at, the metal side is advantageous on the point of not using an adhesive or other material for joining the members, but in addition to the joining step, a step of forming such relief shapes becomes necessary. There was room for improvement in terms of improvement of productivity. As opposed to this, with the method of joining a metal and a plastic member using a laser, an adhesive or other material for joining the members is unnecessary and working the joining surface of the metal also becomes unnecessary. In the method of joining a metal and a plastic using a laser, the following arts of joining members with a high joining strength have been proposed.
PLT 1 discloses the art of jointly using a laser beam source for melting plastic and a laser beam source for breaking down the plastic when joining a metal and a plastic using a laser beam source. In this art, if heating the plastic at the joining location by a single laser beam source, if the distribution of power density cannot be uniformly controlled, large air bubbles and vaporized regions will form at the laser irradiated part, but by melting in advance a broad range of the plastic by the laser beam source for use for melting plastic and directing the laser beam source for use for breaking down the plastic at the narrow location of the melted plastic to break down the plastic, it is possible to create a joining location having a high joining strength at a desired place.
Further, PLT 2 discloses a method of joining a plastic and a metal by firing a laser at their joining surfaces which presses a plastic and a metal from the metal side by a pressing member and makes the pressing member large in light absorption rate and which fires a laser at the pressing member to transfer heat from the pressing member through the metal to the plastic so as to join the plastic and the metal with a high strength and high uniformity.
Further, PLT 3 discloses that it is preferable to press the top part of a joined part by glass etc. since when joining a thermoplastic plastic and a metal by laser, the thermal stress causes the plastic to peel off and therefore excellent joining cannot be realized.
Further, PLT 4 discloses the art of joining two workpieces by laser during which pressing closely together the joining portions of the workpieces by a laser-transparent shaft so as to suppress warping and distortion of the workpieces.
When joining a plastic member and a CFRP member, the arts of adhesive welding, vibration welding, ultrasonic welding, friction welding, and hotplate welding etc. are being used. For example, PLT 5 reports use of the art of friction welding for joining a plastic member and a CFRP member while PLT 6 reports using the art of hotplate welding.
When joining a metal and a CFRP member, there are the method of joining members using an adhesive etc., the method of mechanically joining members using bolts etc., the method of forming fine relief shapes on the bonding surface at the metal side to join members by the anchor effect, etc. (see PLTs 7 and 8).
As opposed to this, in the art of joining a metal and a CFRP member, the art of joining these members by a laser beam has been reported (see NPLT 1).